This photo shows the Millstone nuclear power facility in Waterford, Conn.

HARTFORD, Conn. — Connecticut’s nuclear plant is preparing to ask federal regulators for permission to use water that’s even warmer than the temperature that forced it to shut a unit last August.

Regulators were cool to at least two other suggestions by Millstone Power Station in Waterford to operate with rising water temperatures, according to emails among Nuclear Regulatory Commission officials obtained by The Associated Press through an open records request.

One of the plant’s two operating units was forced to shut down for nearly two weeks last year because the water in the Long Island Sound was warmer than the limit of 75 degrees that’s in place to keep the plant operating safely. The partial shutdown at Millstone was the first in the United States to be caused by rising water temperatures, and the head of the Nuclear Regulatory Commission has asked for a review of climate change impacts on nuclear plants nationwide.

Nuclear plants require large amounts of water to cool equipment and buildings, and federal regulators impose water temperature limits so plants are safely cooled even with water temperatures that are warmer than normal.

Millstone provides half of all power in Connecticut and 12 percent in New England. Its two units produce 2,100 megawatts of electricity, which shrunk 40 percent with its unit down.

As the temperatures rose in the hottest July on record, Millstone, a subsidiary of Dominion Resources Inc., proposed a number of options to get around the temperature requirement. The other Millstone unit, which reaches deeper into the sound, remained open.

The NRC gave Millstone permission to use an average of readings, which brought the measurement down but not enough to avoid the shutdown. Temperatures in the sound were on average 1.7 degrees above the limit.

Millstone also discussed with regulators the possibility of using equipment that more precisely measures water temperature to push the margins out by a few tenths of a degree, according to emails. NRC said that was not a long-term solution.

The plant also discussed whether it could use the average temperature method the entire summer. Regulators “seemed less than enthusiastic” about that prospect, wrote Josephine Ambrosini, an NRC inspector.

“We were pursuing several different avenues at that time,” Millstone spokesman Ken Holt said. “We really started to see the ramp-up in temperature and recognized there was the potential to take the unit offline.”

Millstone is preparing to ask to operate with water at 80 degrees. Holt did not have details on the data being gathered, but Ambrosini told colleagues in an email that the request is expected in the spring at the earliest.

Other nuclear plants have received permission to operate at higher water temperatures, said Diane Screnci, a spokeswoman at the NRC. Millstone will have to “provide a lot of calculations” demonstrating that safety-related equipment could operate with water at higher temperatures, she said.

Weather patterns such as the mild winter in 2011-2012 and little wind that keeps heat in on Long Island Sound were to blame for the unusually warm water, Robert Wilson, a professor at Stony Brook University’s School of Marine and Atmospheric Sciences, said over the summer.

James O’Donnell, a professor of marine sciences at the University of Connecticut, said Long Island Sound water temperatures in the winter of 2012 ranged from nearly 48 degrees in January to about 40 degrees at the end of February.

In contrast, water temperatures this winter have been cooler, ranging from about 44 degrees in January to about 36 degrees this month. O’Donnell characterized it as a normal winter, which could lead to a cooler Long Island Sound this summer than in 2012.

The timing of the shutdown couldn’t have been worse, because demand for power is typically highest in summer as businesses and homes rely on air conditioning.

“You want to be there providing electricity when people are using it the most,” Holt said.

The partial nuclear plant shutdown due to excessively warm water was a first, said NRC spokesman Neil Sheehan. Chairwoman Allison Macfarlane has asked the agency’s staff to look at impacts of climate change on nuclear plants, though it’s focusing on plant changes following the 2011 Fukushima nuclear meltdowns in Japan, he said.

Major impacts from weather included an alert at a New Jersey nuclear plant in October as water from Superstorm Sandy rose outside the plant and threatened cooling equipment.

“We have seen some conditions that did not exist previously and we expect to learn from that going forward,” Sheehan said.